Conventionally, in a process of forming a green tire (tire before vulcanization) for a pneumatic tire, the following method has been known. Specifically, as shown in FIG. 8A, belt plies (b1), a tread rubber (b2), and the like are laminated on a belt drum device (a) to fora a cylindrical belt structure (b) in advance. The belt ply (b1) is formed by winding a sheet-like belt ply in a circumferential direction, for example. The tread rubber (b2) is formed by winding a sheet-like tread rubber material in the circumferential direction, for example.
Further, at this time, a green tire main body (f) is formed by being inflated to have a toroidal shape on a shaping drum (c) positioned adjacently to the belt drum device (a), for example. In the method of forming the green tire main body (f), first, a sheet-like carcass ply (d) is wound in a cylindrical shape on a carcass forming drum (not shown), second, bead cores (e) are attached to both edges of the wound carcass ply to form a carcass base body, then a cylindrical green tire main body (f1) including the carcass base body is formed. Next, by using the shaping drum (c), the green tire main body (f1) is inflated to form the toroidal green tire main body (f). Next, as shown in FIG. 8B, the belt structure (b) is moved onto the green tire main body (f) and bonded together. Next, by using a stitch roller (g), for example, both edges of the belt structure (b) are pressed against the green tire main body (f) so that the entire belt structure (b) is bonded to the green tire main body (f). This process of pressing by the stitch roller (g) may be sometimes referred to as a stitch process.
However, in this stitch process, an outer diameter of the belt structure (b) is remarkably decreased at both edges thereof, resulting in a so-called minus stretch, which leads to undulating deformation, therefore, meandering and arrangement disorder occur in belt cords (not shown) of the belt plies (b1). In particular, if a band ply (not shown) is included in the belt structure (b), since the band cords are continuous in the circumferential direction, meandering becomes large. As described above, it is possible that tires manufactured by the conventional manufacturing method cannot exert high high-speed durability performance for the reasons described above. In order to solve such a problem, for the purpose of suppressing the undulating deformation in the stitch process, it is required that the belt plies (b1) of the belt structure (b) have a shape close to a profile of the belt of the tire after vulcanization in advance of the stitch process.
Japanese Unexamined Patent Application Publication No. 2014-226813 has disclosed a belt drum device for solving the above problem. For example, as shown in FIG. 9A, a belt drum device (h) has an outer peripheral surface (h1) around which the belt structure (b) including the belt plies (b1) is wound, and includes a drum base (i), belt support portions (j), and expansion and contraction means (k). The drum base (i) is formed so as to be rotatable around a drum rotational axis (i1) A plurality (not shown) of the belt support portions (j) is arranged outside the drum base (i) around the drum rotational axis (i1) in a drum circumferential direction. Each of the belt support portions (j) has a central portion (j1) which forms the outer peripheral surface (h1) and extends in a drum axial direction and a pair of side portions (j2) which also form the outer peripheral surface (h1) and are provided on both sides of the central portion (j1). The central portion (j1) is hinge-coupled with each of the side portions (j2) and each of the side portions (j2) is hinge-coupled with the drum base (i). Each of the expansion and contraction means (k) is connected to the central portion (j1) and has a structure to move the central portion (j1) inwardly and outwardly in a drum radial direction. Thereby, each of the expansion and contraction means (k) can expand and contract the outer peripheral surface (h1).
In the belt drum device (h) configured as described above, the outer peripheral surface (h1) can be secured in parallel with the drum rotational axis (i1) of the drum base (i) in a contracted state. Further, as shown in FIG. 9B, in a expanded state, the outer peripheral surface (h1) is deformed so as to protrude outwardly in the drum radial direction. Thereby, in the contracted state, the belt structure (b), especially the sheet-like belt plies (b1) can be wound accurately. Further, by changing from the contracted state to the expanded state, it is possible to make the belt plies (b1) into a toroidal shape convex outwardly in the drum radial direction, therefore, it is possible that the undulating deformation in the stitch process is suppressed.
However, each of the central portion (j1) and the side portions (j2) of the belt drum device (h) is formed in a substantially rectangular shape in a transverse cross-sectional view thereof, thus, in the expanded state, the outer peripheral surface (h1) is formed by a combination of three straight lines, therefore, angular protruding portions (n) are formed at connecting portions between the central portion (j1) and the side portions (j2), thereby, it is difficult to form the belt plies (b1) in a smooth circular arc shape.